Compositionally Engineered Carboxymethyl Cellulose‐Based Optical Fibers With Exceptionally Low Optical Loss and Quantitative Humidity Sensing

ABSTRACT Optical fibers are critical for telecommunications, sensing, and biomedical applications. Still, conventional glass and synthetic polymer optical fibers face limitations in flexibility, biocompatibility, and sustainability. In this context, biopolymer optical fibers (BOFs) offer renewable and biodegradable alternatives for short‐distance applications. However, they suffer from high optical attenuation, limiting their practical use. Here, BOFs derived from carboxymethyl cellulose and its composite with methylcellulose were fabricated via wet spinning using ionic coagulation, achieving an attenuation coefficient of 0.19 dB cm − 1 at 770 nm. This represents the lowest reported attenuation in the literature for cellulose‐based optical fibers. The degree of substitution (DS) in carboxymethyl cellulose influences the fiber optical properties, with DS of 1.2 allowing optimal wet spinning of fibers with circular morphology and tunable optical properties. More importantly, composite fibers obtained by combining carboxymethyl cellulose with the mutually compatible methylcellulose enable quantitative humidity sensing with a sensitivity of up to 0.34 dB %RH −1 . Furthermore, incorporating methylcellulose also enhances telecom O‐band performance and, overall, near‐infrared transmission of BOFs across 800–1350 nm. These findings advance the development of sustainable, biocompatible optical fibers suitable for short‐distance applications and environmental sensing, bridging the gap between biopolymer fibers and commercial polymer optical fibers.